The semiconductor industry, as well as other complex nanotechnology process industries, requires very tight tolerances in process control. As dimensions of chips continue to shrink, the tolerance requirements continue to become tighter. Accordingly, new more precise ways of measuring very small dimensions, e.g., on the order of a few nanometers, are desired. At this scale, typical microscopies, such as optical microscopy, or Scanning Electron Microscopy, are not suitable to obtain the desired precision, or to make quick, non-invasive measurements, which are also desirable.
Optical metrology techniques have been presented as a solution. The basic principle of optical metrology techniques is to reflect and/or scatter light from a target, and measure the resulting light. The received signal can be based simply on the reflectance of the light from the sample, or the change in polarization state (Psi, Delta) of the light caused by the sample. The light may be modeled to retrieve the geometries or other desired parameters of the illuminated sample.
With the continuously shrinking of critical dimensions in semiconductor devices and the industry moving to 3D device structures, optical metrology that is used to measure device structure parameters is facing more and more challenges due to lack of parameter sensitivity or strong parameter correlations. Continued improvements in optical metrology are therefore desirable.